Method for removing silicon oxide film and processing apparatus

ABSTRACT

A silicon dioxide film removing method is capable of removing a silicon dioxide film, such as a natural oxide film or a chemical oxide film, at a temperature considerably higher than a room temperature. The silicon dioxide film removing method of removing a silicon dioxide film formed on a workpiece in a processing vessel  18  that can be evacuated uses a mixed gas containing HF gas and NH 3  gas for removing the silicon dioxide film. The silicon dioxide film can be efficiently removed from the surface of the workpiece by using the mixed gas containing HF gas and NH 3  gas.

TECHNICAL FIELD

The present invention relates to a method of removing a silicon dioxidefilm formed on a surface of a workpiece, such as a semiconductor wafer,and a processing system.

BACKGROUND ART

Generally, a semiconductor wafer, such as a silicon substrate, issubjected to various processes including a film forming process, anetching process, an oxidizing process, a diffusion process andmodification process to fabricate a semiconductor integrated circuit onthe semiconductor wafer. When the semiconductor wafer is carried fromone processing vessel for the preceding process to another processingvessel for the succeeding process, the semiconductor wafer is exposed toa clean atmosphere. Oxygen and moisture contained in the atmospherereact with active silicon atoms exposed on the surface of the wafer andform a natural oxide film of SiO₂. The natural oxide film deteriorateselectrical properties. Therefore, the natural oxide film is removed by awet cleaning process using, for example, an HF solution beforesubjecting the semiconductor wafer to the next process. A film removingmethod disclosed in Patent document 1 removes a silicon dioxide film ofa different quality selectively with HF gas at a room temperature.

The surface of the wafer from which the natural oxide film has beenremoved by the cleaning process is highly active. Therefore, if thewafer is exposed to the atmosphere, a natural oxide film (SiO₂ film) isformed again. To avoid the reformation of a natural oxide film on thewafer, the wet surface of the wafer from which the natural oxide filmhas been removed is processed by a chemical process to form a chemicaloxide film (SiO₂ film) on the cleaned surface of the wafer, the wafercoated with the chemical oxide film is carried to the processing vesselof the next process, and the wafer coated with the chemical oxide filmis processed. The chemical oxide film has excellent electricalproperties as compared with the natural oxide film and is formeduniformly on the surface of the wafer. Therefore, when the next processis a gate oxide film forming process, a thermal oxide film (SiO₂ film)is formed directly on the wafer.

A series of steps of the foregoing processing method of processing thesurface of a semiconductor wafer will be described with reference toFIG. 10. It is supposed that the processing method is applied to forminga thermal oxide film (SiO₂ film), for example, to be used as a gateoxide film on the surface of a semiconductor wafer.

Referring to FIG. 10(A), a surface of a semiconductor wafer W, such as asilicon substrate, is exposed to the atmosphere. Consequently, a naturaloxide film 2 (SiO₂ film) having an irregular thickness and inferiorelectrical properties is formed on a surface of the semiconductor waferW through the interaction of oxygen and steam (moisture) contained inthe atmosphere and silicon atoms. As shown in FIG. 10(B), thesemiconductor wafer W is subjected to a wet cleaning process using an HFsolution to remove the natural oxide film 2 from the surface of thesemiconductor wafer W, the surface of the wafer W exposed by removingthe natural oxide film 2 is highly active and a natural oxide film isreadily formed again on the wafer W.

To prevent the reformation of a natural oxide film on the wafer W, thewafer W is subjected to a chemical process using a solution prepared bymixing, for example, H₂O₂ and NH₄OH after the removal of the naturaloxide film 2 to form a protective chemical oxide film (SiO₂ film) 4 bylightly oxidizing the surface of the wafer W as shown in FIG. 10(C). Thechemical oxide film 4 is superior in electrical properties to thenatural oxide film 2 and is uniform and very thin. The thickness L ofthe chemical oxide film 4 is in the range of about 0.7 to about 0.9 nm.

Then, as shown in FIG. 10(D), the wafer W is carried to, for example, athermal oxidation system and is subjected to a thermal oxidation processto form a thermal oxide film (SiO₂ film) 6 (refer to, for example,Patent documents 2 and 3). The thermal oxide film 6 is processed by apatterning etching process to use the thermal oxide film 6 as a gateoxide film. The thermal oxide film 6 is sandwiched between the siliconsurface of the wafer W and the chemical oxide film 4.

Patent document 1: JP 6-181188 A

Patent document 2: JP 3-140453 A

Patent document 3: JP 2002-176052

The thickness of a film in one layer has been progressively decreased tocope with the further advancement of the complexity of integration anddimensional reduction of semiconductor integrated circuits. Under suchcircumstances, it is desired to control skillfully processes forforming, for example, gate oxide films to form gate oxide films in adesired thickness in the range of, for example, 1.0 to 1.2 nm.

Although the chemical oxide film 4 has the very small thickness L in therange of about 0.7 to about 0.9 nm, the ratio of the thickness L of thechemical oxide film to the thickness of the gate oxide film, namely, thesum of the respective thicknesses of the chemical oxide film 4 and thethermal oxide film 6, increases when the desired thickness of the gateoxide film is in the range of about 1.0 to about 1.2 nm. The control ofthe thickness of such a gate oxide film is difficult. Such a problemresides not only in forming the gate oxide film, but also in formingvarious kinds of thin film.

The chemical oxide film may be removed by the film removing method usingHF gas as mentioned in Patent document 1. However, a process singlyusing HF gas needs to be carried out at a room temperature. It takesmuch time to change the temperature of a processing vessel, namely, avertical furnace, having a large heat capacity in a wide temperaturerange, which reduces throughput greatly.

DISCLOSURE OF THE INVENTION

The present invention has been made in view of the foregoing problems tosolve those problems. Accordingly, it is an object of the presentinvention to provide a silicon dioxide film removing method and aprocessing system capable of efficiently removing a silicon dioxidefilm, such as a natural oxide film or a chemical oxide film, in anatmosphere of a temperature considerably higher than a room temperature.

According to the present invention, a silicon dioxide film removingmethod of removing a silicon dioxide film formed on a surface of aworkpiece in a processing vessel that can be evacuated is characterizedin using a mixed gas containing HF gas and NH₃ gas for removing thesilicon dioxide film.

The silicon dioxide film formed on the surface of the workpiece can beefficiently removed by using the mixed gas containing HF gas and NH₃gas.

In the silicon dioxide film removing method, a processing temperature atwhich the workpiece is processed is, for example, in the range of 100°C. to 600° C.

In the silicon dioxide film removing method, a processing pressure atwhich the workpiece is processed is in the range of, for example, 26 to53,200 Pa (0.2 to 400 torr).

In the silicon dioxide film removing method, the silicon dioxide filmis, for example, a chemical oxide film formed by a chemical process, anda processing temperature for achieving etch selectivity for the chemicaloxide film to silicon is in the range of, for example, 100° C. to 400°C.

The silicon dioxide film, namely, the chemical oxide film, can be etchedand removed by etching at a high degree of etch selectivity.

In the silicon dioxide film removing method, the processing pressure isin the range of, for example, 26 to 53,200 Pa (0.2 to 400 torr).

In the silicon dioxide film removing method, the flow rate ratio of HFgas to NH₃ gas is in the range of, for example, 10:1 to 1:50.

In the silicon dioxide film removing method, the silicon dioxide filmis, for example, a chemical oxide film formed by a chemical process, anda processing temperature for achieving etch selectivity for the chemicaloxide film to a silicon nitride film is in the range of, for example,200° C. to 600° C.

The silicon dioxide film, namely, the chemical oxide film, can be etchedand removed at a high degree of etch selectivity for the chemical oxidefilm to the silicon nitride film.

In the silicon dioxide film removing method, the silicon dioxide filmis, for example, a chemical oxide film formed by a chemical process, anda processing temperature for achieving etch selectivity for the chemicaloxide film to a silicon dioxide film formed by decomposing TEOS(tetraethylorthosilicate), which will be referred to as “TEOS silicondioxide film”, is in the range of, for example, 300° C. to 400° C.

The chemical oxide film can be etched and removed at a high degree ofetch selectivity for the chemical oxide film to the TEOS silicon dioxidefilm.

In the silicon dioxide film removing method, The silicon dioxide film isa chemical oxide film formed by a chemical process, and a processingtemperature for achieving etch selectivity for the chemical oxide filmto the thermal oxide film is in the range of, for example, 100° C. to600° C.

The chemical oxide film, namely, the silicon dioxide film, can be etchedand removed at a high degree of etch selectivity for the chemical oxidefilm to the thermal oxide film (SiO₂).

In the silicon dioxide film removing method, the flow rate ratio of HFgas to NH₃ gas is in the range of, for example, 1:10 to 1:50.

In the silicon dioxide film removing method, the processing pressure is,for example, 1011 Pa (7.6 torr) or below.

In the silicon dioxide film removing method, the silicon dioxide filmis, for example, a natural oxide film.

According to the present invention, a processing system for carrying outthe foregoing silicon dioxide film removing method includes: aprocessing vessel that can be evacuated; a workpiece holding means forholding workpieces; a heating means for heating the workpieces; anevacuating system for evacuating the processing vessel; an HF gas supplysystem for supplying HF gas into the processing vessel; and an NH₃ gassupply system for supplying NH₃ gas into the processing vessel.

The processing system may further include an oxidizing gas supply systemfor supplying steam or a gas for generating steam into the processingvessel.

The processing system may further include a silicon film forming gassupply system for supplying a silicon film forming gas into theprocessing vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a processing system for carryingout a silicon dioxide film removing method in a first embodimentaccording to the present invention;

FIG. 2 is a view of assistance in explaining steps of a semiconductorwafer processing process;

FIG. 3 is a graph showing changes in the thickness of chemical oxidefilms caused by an etching process;

FIG. 4 is a graph showing the NH₃ gas dependence of thickness reductionin chemical oxide films;

FIG. 5 is a table showing data on etch selectivity for a chemical oxidefilm to silicon dioxide films other than the chemical oxide films, andfilms of silicon-containing materials;

FIG. 6 is a table showing data on etch selectivities for chemical oxidefilms, silicon dioxide films other than the chemical oxide films, andfilms of silicon-containing materials;

FIG. 7 is a bar graph showing the data on TOP and BTM wafers shown inFIG. 6;

FIG. 8 is a schematic sectional view of a processing system including anoxidizing gas supply system for supplying steam or gases for generatingsteam;

FIG. 9 is a schematic sectional view of a processing system including asilicon film forming gas supply system for supplying gases for forming asilicon film; and

FIG. 10 is a view of assistance in explaining a series of steps of aprocess for processing a surface of a semiconductor wafer.

BEST MODE FOR CARRYING OUT THE INVENTION

A silicon dioxide film removing method in a first embodiment accordingto the present invention and processing systems for carrying out thesame will be described with reference to the accompanying drawings.

FIG. 1 shows a processing system 12 for carrying out a silicon dioxidefilm removing method in a preferred embodiment according to the presentinvention. The processing system 12 includes a double-wall, verticalprocessing vessel 18 of a predetermined length. The processing vessel 18includes an inner tube 14 of quartz and an outer tube 16 of quartz. Theinner tube 14 defines a processing space S. A wafer boat 20 of quartz,namely, a workpiece holding means, is placed in the processing space S.Semiconductor wafers W, namely, workpieces, are held in layers on thewafer boat 20 at predetermined vertical pitches. The pitches may beequal or may be different according to the positions of the wafers W.

A cap 22 closes and opens the lower end of the processing vessel 18. Arotating shaft 26 penetrates the cap 22. A gap between the cap 22 andthe rotating shaft 26 is sealed with a magnetic fluid seal 24. Arotating table 28 is supported on the rotating shaft 26. A heatinsulating cylinder 30 is mounted on the table 28. The wafer boat 20 isplaced on the heat insulating cylinder 30. The rotating shaft 26 issupported on a vertically movable arm 34 included in a boat elevator 32.The rotating shaft 26, the cap 22 and the wafer boat 20 can besimultaneously moved in vertical directions. The wafer boat 20 does notneed necessarily to be rotated and may be fixed.

A manifold 36 made of, for example, a stainless steel is joined to theopen lower end of the processing vessel 18. An HF gas supply system 38and an NH₃ gas supply system 40 are connected individually to themanifold 36. The HF gas supply system 38 and the NH₃ gas supply system40 supply HF gas and NH₃ gas at controlled flow rates, respectively,into the processing vessel 18.

More specifically, the HF gas supply system 38 includes an HF gas nozzle42 penetrating the manifold 36. A gas supply line 46 provided with aflow controller 44, such as a mass flow controller, has one endconnected to the HF gas nozzle 42 and the other end connected to an HFgas source 48.

The NH₃ gas supply system 40 includes an NH₃ gas nozzle 50 penetratingthe manifold 36. A gas supply line 54 provided with a flow controller52, such as a mass flow controller, has one end connected to the NH₃ gasnozzle 50 and the other end connected to an NH₃ gas source 56.

HF gas and NH₃ gas supplied through the nozzles 42 and 50 into theprocessing vessel 18 flow upward in the processing space S, namely, awafer holding space. The flow of the HF gas and the NH₃ gas is turned bythe top wall of the processing vessel 18 such that the HF gas and theNH₃ gas flow downward through an annular space defined by the inner tube14 and the outer tube 16 and are discharged through a discharge port 58formed in a lower part of the outer tube 16. The discharge port 58 isconnected to a vacuum discharge system 64 including a discharge line 60and a vacuum pump 62. The vacuum discharge system 64 evacuates theprocessing vessel 18.

The processing vessel 18 is surrounded by a heat insulating wall 66. Aheater 68, namely, a heating means, is installed on the inner surface ofthe heat insulating wall 66. The heater 68 heats the wafers W held inthe processing vessel 18 at a predetermined temperature. Suppose thatthe wafers W are 8 in. wafers and the number of the wafers W includingabout twenty dummy wafers held on the wafer boat 20 is on the order ofone hundred and fifty. Then, the diameter of the inner tube 14 is in therange of about 260 to about 270 mm, the diameter of the outer tube 16 isin the range of about 275 to about 285 mm and the height of theprocessing vessel 18 is on the order of 1280 mm.

When the wafers W are 12 in. wafers, and the number of the wafers W heldon the wafer boat 20 is in the range of about 25 to about 50, thediameter of the inner tube 14 is in the range of about 380 to about 420mm, the diameter of the outer tube 16 is in the range of about 440 toabout 500 mm and the height of the processing vessel 18 is on the orderof 800 mm. Those numerical values are only examples.

A sealing member 70, such as an O ring, is held between the cap 22 andthe manifold 36. A sealing member 72, such as an O ring, is held betweenthe upper end of the manifold 36 and the lower end of the outer tube 16.Needless to say that the processing system is provided with an inert gassupply system, not shown, for supplying an inert gas, such as N₂ gas.

A silicon dioxide film removing method to be carried out by theprocessing system will be described.

As shown in FIG. 2(A), a chemical oxide film 4, namely, a silicondioxide film, is formed on a surface of a semiconductor wafer W. Asmentioned above in connection with FIGS. 8(A) and 8(B), the chemicaloxide film 4 is formed on the surface of the wafer W after removing anatural oxide film 2 from the surface of the wafer W by a chemicalprocess using a mixed solution of H₂O₂ and NH₄OH.

The processing system 12 processes the wafer W with the chemical oxidefilm 4 by an etching process using a mixed gas containing HF gas and NH₃gas to remove the chemical oxide film 4 as shown in FIG. 2(B).

Then, a thermal oxide film 6 for forming, for example, a gate oxide filmis formed on the wafer W as shown in FIG. 2(C) by another processingsystem. A silicon dioxide film removing method in a second embodiment,which will be described later, carries out both a film removing processfor removing the chemical oxide film 4 and a film forming process forforming the thermal oxide film 6 by a single processing system.

A film removing method of removing the chemical oxide film 4 by theprocessing system 12 will be more specifically described.

As shown in FIG. 2(A), a plurality of semiconductor wafers W each havinga surface coated with a chemical oxide film 4 are arranged in layers atpredetermined pitches on the wafer boat 20. The boat elevator 32 raisesthe wafer boat 20 upward to load the wafer boat 20 through the lower endof the processing vessel 18 into the processing vessel 18. Then, theprocessing vessel 18 is sealed. The interior of the processing vessel 18is heated beforehand at a predetermined temperature. The surfaces of thesemiconductor wafers W are coated with the chemical oxide films 4,respectively. Then, the vacuum discharge system 64 is operated toevacuate the processing vessel 18.

At the same time, HF gas is supplied into the processing vessel 18 at acontrolled flow rate through the HF gas nozzle 42 of the HF gas supplysystem 38, and NH₃ gas is supplied into the processing vessel 18 at acontrolled flow rate through the NH₃ gas nozzle 50 of the NH₃ gas supplysystem 40.

The HF gas and the NH₃ gas separately supplied into the processingvessel mix together to form a mixed gas as the HF gas and the NH₃ gasflow upward in the processing vessel 18. The mixed gas etches andremoves the chemical oxide films 4 of the wafers W.

An etching process using this mixed gas is carried out at a processingtemperature higher than a room temperature, such as a temperature in therange of 100° C. to 600° C., a processing pressure in the range of, forexample, 26 to 53,200 Pa (0.2 to 400 torr).

The silicon dioxide film cannot be removed unless the temperature in theprocessing vessel 18 is decreased to a temperature near a roomtemperature when only HF gas is used. The silicon dioxide film, namely,the chemical oxide film, can be removed without decreasing thetemperature in the processing vessel 18 to a temperature near a roomtemperature when the mixed gas containing HF gas and NH₃ gas.Consequently, the temperature in the processing vessel 18 can beadjusted in a short time and hence throughput can be increased.

After the chemical oxide films 4 have been removed, a thermal oxide film6, such as a gate oxide film, is formed on each of the wafers W as shownin FIG. 2(C) by an oxidation process by the processing system 12 whenthe processing system 12 is provided with members for forming a thermaloxide film. When the processing system 12 is not provided with membersfor forming a thermal oxide film, the wafers W are carried to anotherprocessing system provided with members for forming a thermal oxide filmand a thermal oxide film 6 is formed on each of the wafers W by anoxidation process. Even if the gate oxide film needs to be formed in asmall thickness in the range of about 1.0 to about 1.2 nm to cope withthe further advancement of the complexity of integration and dimensionalreduction of semiconductor integrated circuits as mentioned above, thethickness of the thermal oxide film 6 can be satisfactorily controlled.

Results of evaluation of changes in the thickness of the chemical oxidefilms (SiO₂ films) will be described. FIG. 3 is a graph showing changesin the thickness of the chemical oxide films. In FIG. 3, “TOP” and “BTM”indicate the wafers W at a top position and a bottom position,respectively, on the wafer boat 20. The chemical oxide films were etchedby an etching process. The etching process used a processing temperatureof 300° C., a processing pressure of 53,200 Pa (400 torr), an HF gasflow rate of 182 sccm, an NH₃ gas flow rate of 1,820 sccm, a N₂ gas flowrate of 8,000 sccm and an etching time of 10 min.

As obvious from FIG. 3, the respective thicknesses of the chemical oxidefilms of the TOP wafer W and the BTM wafer W are reduced greatly, andthe thicknesses of parts, removed in 10 min by etching, of the chemicaloxide films were in the range of about 0.39 to about 0.41 nm.

The effect of the mixed gas containing NH₃ gas was examined. Results ofexamination will be explained. FIG. 4 is a graph showing the NH₃ gasdependence of thickness reduction in chemical oxide films. In FIG. 4,thickness reductions when only HF gas was used are shown on the leftside and those when the mixed gas containing HF gas and NH₃ gas areshown on the right side. The chemical oxide films were etched by anetching process. The etching process used a processing temperature of300° C., a processing pressure of 53,200 Pa (400 torr), an HF gas flowrate of 182 sccm, an NH₃ gas flow rate of 1,820 sccm, a N₂ gas flow rateof 8,000 sccm and an etching time of 10 min.

As obvious from FIG. 4, whereas the chemical oxide films were scarcelyetched when only HF gas was used, the chemical oxide films were etchedby thicknesses in the range of about 0.59 to about 0.61 nm when themixed gas containing HF gas and NH₃ gas was used. Thus it is known thatthe chemical oxide films cannot be etched unless the etching gascontains NH₃ gas.

Selectivities for chemical oxide films, silicon dioxide films other thanchemical oxide films and films of silicon-containing materials wereexamined. FIG. 5 is a table showing data on selectivities for chemicaloxide films, silicon dioxide films other than chemical oxide films, andsilicon-containing materials. Unit of processing pressure is torr (1torr=133 Pa). In FIG. 5, circles indicate effective selectivity and amark “−” signifies that measurement was impossible due to theoveretching of Si.

The processing temperature was varied in the range of 100° C. to 600° C.and the processing pressure was varied in the range of 26 to 53,200 Pa(0.2 to 400 torr). The duration of the etching process was 10 min. InFIG. 5, “VAC” indicates a pressure in the evacuated processing vessel 18in the range of about 26 to about 40 Pa (about 0.2 to about 0.3 torr),which is dependent on the ability of the vacuum pump. The flow rateratio of HF gas to NH₃ gas was varied in the range of 1:10 to 10:1. InFIG. 5, numerical values are thicknesses in nanometer of parts of thefilms removed in 10 min by the etching process.

Chemical oxide films, silicon films (polysilicon films), silicon nitridefilms (SiN films), TEOS silicon dioxide films, and thermal oxide filmsformed by thermal oxidation were examined.

Data on the chemical oxide films shows that, although the thicknesses ofetched layers of the chemical oxide films were different, the chemicaloxide films could be etched at processing temperatures in the range of100° C. to 600° C. and at processing pressures in the range of VAC (0.2torr) to 400 torr. The chemical oxide films could be etched when theprocessing pressure was VAC and a NH₃-rich mixed gas was used, even ifthe processing temperature was 100° C. Under a condition where theprocessing temperature was 100° C. and the processing pressure was 7.6torr or 400 torr and a condition where the processing temperature was300° C. and the processing pressure was VAC, the chemical oxide filmscould not be etched because the mixed gas was not NH₃-rich. The chemicaloxide films could not be etched at all by an etching process using aprocessing temperature of 50° C., namely, a temperature lower than 100°C., and a mixed gas containing HF gas and NH₃ gas. It is known that theprocessing temperature must be 100° C. or above.

Etch selectivity for the chemical oxide film to polysilicon (Si) will beexamined.

As obvious from FIG. 5, the polysilicon films are etched at high etchrates under a condition where the processing temperature is 100° C. andthe processing pressure is 400 torr and a condition where the processingtemperature is 600° C. and the processing pressure is VAC. Under acondition where the processing temperature is 300° C. or 400° C., theprocessing temperature is in the range of VAC to 400 torr and the flowrate ratio of HF gas to NH₃ gas is in the range of 10:1 to 1:10, thepolysilicon films are not etched at all. Thus it is known that thechemical oxide film can be selectively etched relative to polysiliconwhen the processing temperature is in the range of 300° C. to 400° C.

Etch selectivity for the chemical oxide film to the silicon nitride filmwill be examined.

As obvious from FIG. 5, the silicon nitride films are etched when theprocessing temperature is in the range of 100° C. to 600° C. The siliconnitride films are etched at etch rates lower than those at which thechemical oxide films are etched under a condition where the processingtemperature is 300° C. and the processing pressure is 7.6 torr, acondition where the processing temperature is 400° C. and the processingpressure is VAC or 7.6 torr and a condition where the processingtemperature is 600° C. and the processing pressure is VAC. Thus it isknown that the chemical oxide films can be selectively etched relativeto the silicon nitride films when the processing temperature is in therange of 300° C. to 600° C. and the processing pressure is 7.6 torr orbelow.

Etch selectivity for the chemical dioxide film to the TEOS silicondioxide film will be examined.

As obvious from FIG. 5, the TEOS silicon dioxide films are etched whenthe processing temperature is in the range of 100° C. to 600° C. TheTEOS silicon dioxide films are etched at etch rates lower than those atwhich the chemical oxide films under a condition where the processingtemperature is 300° C. and the processing pressure is 7.6 torr, and acondition where the processing temperature is 400° C. and the processingpressure is VAC or 7.6 torr. Thus it is known that the chemical oxidefilms can be selectively etched relative to the TEOS silicon dioxidefilms when the processing temperature is in the range of 300° C. to 400°C. and the processing pressure is 7.6 torr or below.

Etch selectivity for the chemical dioxide film to the thermal oxide film(SiO₂ film) formed by thermal oxidation will be examined.

As obvious from FIG. 5, the silicon dioxide films formed by thermaloxidation are etched when the processing temperature is in the range of100° C. to 600° C. The thermal oxide films formed by thermal oxidationare etched at etch rates lower than those at which the chemical oxidefilms under a condition where the processing temperature is 100° C. andthe processing pressure is VAC, a condition where the processingtemperature is 300° C. and the processing pressure is 7.6 torr, 150 torrpr 400 torr, namely a condition where the mixed gas if NH₃-rich, acondition where the processing temperature is 400° C. and the processingpressure is in the range of VAC to 400 torr, and a condition where theprocessing temperature is 600° C. and the processing pressure is VAC or7.6 torr. Thus it is known that the chemical oxide films can beselectively etched relative to the silicon dioxide films formed bythermal oxidation when the processing temperature is in the range of100° C. to 600° C.

As obvious from FIG. 5, the mixed gas containing HF gas and NH₃ gas iscapable of etching not only the chemical oxide films, but also the TEOSsilicon dioxide films and the silicon dioxide films formed by thermaloxidation. Thus the mixed gas is capable of etching silicon dioxidefilms other than the foregoing silicon dioxide films, such as naturaloxide films formed on silicon wafers and silicon dioxide films depositedon wafers by thermal CVD processes and plasma CVD processes.

Etch selectivity of NH₃-rich mixed gas containing HF gas and NH₃ gaswill be explained.

FIG. 6 is a table showing data on etch selectivities for chemical oxidefilms, silicon dioxide films other than the chemical oxide films, andfilms of silicon-containing materials, and FIG. 7 is a bar graph showingdata on TOP and BTM wafers shown in FIG. 6.

FIG. 6 shows data on films formed by using mixed gases richer in NH₃ gasthan the mixed gases used for forming the films having propertiesrepresented by data shown in FIG. 5. HF gas and NH₃ gas were supplied atHF:NH₃ flow rate ratios of 1:10, 1:20 and 1:50. The processingtemperature was 200° C., and the processing pressure was 150 torr, whichwere the means of the processing temperatures and processing pressuresshown in FIG. 5, respectively. The processing time was 10 min. The flowrate of NH₃ gas was fixed at 1,820 sccm and the flow rate of HF gas waschanged to change the HF:NH₃ flow rate ratio. One hundred and fiftywafers were used.

As obvious from FIGS. 6 and 7, whereas the polysilicon films were etchedscarcely when HF gas and NH₃ gas were supplied at flow rate ratios of1:10, 1:20 and 1:50, the chemical oxide films were etched stably and thethicknesses of etched layers of the chemical oxide films were in therange of 0.41 to 0.57 nm.

As obvious from FIGS. 6 and 7, the higher the etch selectivity for thechemical oxide film t the SiN film, the TEOS silicon dioxide film andthe thermal oxide film, the smaller is the thickness of the etched layerof the chemical oxide film.

The thicknesses of the etched layers of the SiN film and the thermaloxide film are substantially equal to or greater than that of the etchedlayer of the chemical oxide film when the HF:NH₃ flow rate ratio is1:10. The thicknesses of the etched layers of the SiN film and thethermal oxide film are considerably smaller than that of the etchedlayer of the chemical oxide film when the HF:NH₃ flow rate ratio is1:20. The SiN film and the thermal oxide film are etched scarcely whenthe HF:NH₃ flow rate ratio is 1:50.

It is known from the foregoing facts that it is effective to use themixed gas having the highest possible NH₃ concentration to etch thechemical oxide film efficiently while the etching of the SiN film, theTEOS silicon dioxide film and the thermal oxide film is controlled. Itis known that a desirable HF:NH₃ flow rate ratio is in the range of 1:20to 1:50

Although the processing system shown by way of example in FIG. 1 isprovided with only the HF gas supply system 38 and the NH₃ gas supplysystem 40 to facilitate understanding, the processing system may beadditionally provided with other gas supply systems for other processes.FIG. 8 shows a processing system provided with an oxidizing gas supplysystem for supplying steam or gases for generating steam. The processingsystem will be described with reference to FIG. 8, in which parts likeor corresponding to those shown in FIG. 1 are denoted by the samereference characters and the description thereof will be omitted.

Referring to FIG. 8, the processing system includes, in addition to anHF gas supply system 38 and an NH₃ gas supply system 40, an oxidizinggas supply system 80. The oxidizing gas supply system 80 has an H₂ gassource 80A and an O₂ gas source 80B. The H₂ gas source 80A and the O₂gas source 80B are connected by gas supply lines 84A and 84Brespectively provided with flow controllers 82A and 82B to gas nozzles86A and 86B, respectively. H₂ gas and O₂ gas are supplied through thegas nozzles 86A and 86B, respectively, into a processing vessel 18.

In the processing vessel 18, H₂ gas and O₂ gas interact to generatesteam. Surfaces of silicon wafers are processed by a thermal oxidationprocess using the steam to form a thermal oxide film on each of thesilicon wafers.

First, the processing system shown in FIG. 8 supplies HF gas and NH₃ gasinto the processing vessel 18 to remove chemical oxide films coveringthe surfaces of the silicon wafers. Then, the processing system suppliesH₂ gas and O₂ gas into the processing vessel 18 to generate steam afterstopping supplying HF gas and NH₃ gas. Then, the processing systemcarries out a thermal oxidation process using the steam to form athermal oxide film as a gate oxide film on each of the silicon wafers.

The oxidizing gas supply system 80 may include an external combustor ora steam generator using a catalyst to generate steam and may supplyexternally generated steam into the processing vessel 18.

A processing system including a silicon film forming gas supply systemwill be described with reference to FIG. 9, in which parts like orcorresponding to those shown in FIG. 1 are denoted by the same referencecharacters and the description thereof will be omitted.

Referring to FIG. 9, the processing system includes, in addition to anHF gas supply system 38 and an NH₃ gas supply system 40, a silicon filmforming gas supply system 90. The silicon film forming gas supply system90 has a SiH₄ gas source 90A. The SiH₄ gas source 90A is connected to agas nozzle 96A by a gas supply line 94A provided with a flow controller92A. SiH₄ gas is supplied through the gas nozzle 96A into the processingvessel 18 when needed.

The processing system is provided with a GeH₄ gas source 90B to supply adopant into the processing vessel 18. The GeH₄ gas source 90B isconnected to a gas nozzle 96B by a gas supply line 94B provided with aflow controller 92B. GeH₄ gas is supplied through the gas nozzle 96Binto the processing vessel 18 when needed.

A silicon film (polysilicon film) doped with germanium, namely, adopant, can be formed by supplying SiH₄ gas and GeH₄ gas into theprocessing vessel 18.

First, the processing system shown in FIG. 9 supplies HF gas and NH₃ gasinto the processing vessel 18 to remove chemical oxide films coveringthe surfaces of the silicon wafers. Then, the processing system suppliesSiH₄ gas and GeH₄ gas into the processing vessel 18 to form a siliconfilm doped with germanium, namely, a dopant. Thus, the processing systemshown in FIG. 9 is capable of continuously carrying out processes forremoving a chemical oxide film and forming a doped silicon film. Anepitaxial film doped with germanium can be formed by properlydetermining a processing temperature. A processing system capable ofcontinuously carrying out a chemical oxide film removing process, a gateoxide film forming process and a silicon gate electrode forming processcan be constructed by combining the processing systems shown in FIGS. 8and 9.

Although processes for forming a thermal oxide film and a silicon filmdoped with germanium to be carried out after removing a chemical oxidefilm have been described by way of example, a metal film, a nitride filmor an insulating film may be formed after removing a chemical oxidefilm. The foregoing processes may be continuously carried out afterremoving, for example, a thermal oxide film instead of a chemical oxidefilm by the method according to the present invention.

The foregoing processing system in the embodiment including thedouble-wall processing vessel is only an example. The present inventionis applicable to, for example, a processing system including asingle-wall processing vessel. Each process gas is supplied through thelower end (or the upper end) of the single-wall processing vessel andthe processing vessel is evacuated through the upper end (or the lowerend) of the single-wall processing vessel.

The present invention is not limited to a batch type processing vesselcapable of simultaneously processing a plurality of semiconductor wafersby an oxidation process. The present invention is applicable to asingle-wafer processing system that places a single wafer on a supporttable (support means) placed in a processing vessel and heats the waferwith a heating means, such as a heating lamp or a heating device, toprocess the wafer by an oxidation process.

The workpieces are not limited to semiconductor wafers; the processingsystem of the present invention is applicable to processing LCDsubstrates, glass substrates and such.

As apparent from the foregoing description, the silicon dioxide filmremoving method and the processing system according to the presentinvention has the following effects.

A silicon dioxide film formed on a surface of a workpiece can beefficiently removed by using a mixed gas containing HF gas and NH₃ gas.

A chemical oxide film can be etched at high etch selectivity to siliconand can be efficiently removed.

A chemical oxide film can be etched at high etch selectivity to siliconnitride film and can be efficiently removed.

A silicon dioxide oxide film, namely, chemical oxide film, can be etchedat high selectivity to TEOS silicon dioxide film and can be efficientlyremoved.

A silicon dioxide film, namely, a chemical oxide film, can be etched athigh etch selectivity to a thermal oxide film (SiO₂) and can beefficiently removed.

1. A method of removing silicon dioxide films from a surface of aworkpiece having a natural oxide film thereon, said method comprising:removing the natural oxide film from the surface of the workpiece by achemical process; forming a chemical oxide film on the surface as aprotective film after removal of the natural oxide film, the chemicaloxide film being a silicon dioxide film formed by a chemical processusing a solution prepared by mixing H₂O₂ and NH₄OH; and removing thechemical oxide film from the surface of the workpiece, under conditionsthat: a mixed gas containing HF gas and NH₃ gas is used; a processingtemperature for achieving etch selectivity for the chemical oxide filmto silicon is in the range of 200° C. to 400° C.; a processing pressureat which the workpiece is processed is in the range of 26 Pa (0.2 Torr)to 53,200 Pa (400 Torr); and the flow rate ratio of HF gas to NH₃ gas isin the range of 10:1 to 1:50.
 2. A method of removing silicon dioxidefilms from a surface of a workpiece having a natural oxide film thereon,said method comprising: removing the natural oxide film from the surfaceof the workpiece by a chemical process; forming a chemical oxide film onthe surface as a protective film after removal of the natural oxidefilm, the chemical oxide film being a silicon dioxide film formed by achemical process using a solution prepared by mixing H₂O₂ and NH₄OH; andremoving the chemical oxide film from the surface of the workpiece,under conditions that: a mixed gas containing HF gas and NH₃ gas isused; a processing temperature for achieving etch selectivity for thechemical oxide film to a silicon nitride film is in the range of 200° C.to 600° C.; a processing pressure at which the workpiece is processed isnot more than 53,200 Pa (400 Torr); and the flow rate ratio of HF gas toNH₃ gas is in the range of 10:1 to 1:50.
 3. A method of removing silicondioxide films from a surface of a workpiece having a natural oxide filmthereon, said method comprising: removing the natural oxide film fromthe surface of the workpiece by a chemical process; forming a chemicaloxide film on the surface as a protective film after removal of thenatural oxide film, the chemical oxide film being a silicon dioxide filmformed by a chemical process using a solution prepared by mixing H₂O₂and NH₄OH; and removing the chemical oxide film from the surface of theworkpiece under conditions that: a mixed gas containing HF gas and NH₃gas is used; a processing temperature for achieving etch selectivity forthe chemical oxide film to a silicon dioxide film, which has been formedby CVD (Chemical Vapor Deposition), is in the range of 200° C. to 400°C.; a processing pressure at which the workpiece is processed is notmore than 53,200 Pa (400 Torr); and the flow rate ratio of HF gas to NH₃gas is in the range of 10:1 to 1:50.
 4. A method of removing silicondioxide films from a surface of a workpiece having a natural oxide filmthereon, said method comprising: removing the natural oxide film fromthe surface of the workpiece by a chemical process; forming a chemicaloxide film on the surface as a protective film after removal of thenatural oxide film, the chemical oxide film being a silicon dioxide filmformed by a chemical process using a solution prepared by mixing H₂O₂and NH₄OH; and removing the chemical oxide film from the surface of theworkpiece, under conditions that: a mixed gas containing HF gas and NH₃gas is used; a processing temperature for achieving etch selectivity forthe chemical oxide film to a thermal oxide film is in the range of 100°C. to 600° C.; a processing pressure at which the workpiece is processedis not more than 53,200 Pa (400 Torr); and the flow rate ratio of HF gasto NH₃ gas is in the range of 10:1 to 1:50.
 5. The method of claim 1,further comprising: forming a thermal oxide film on the surface afterremoval of the chemical oxide film, whereby the thermal oxide film isthe only film on the surface.
 6. The method of claim 2, furthercomprising: forming a thermal oxide film on the surface after removal ofthe chemical oxide film, whereby the thermal oxide film is the only filmon the surface.
 7. The method of claim 3, further comprising: forming athermal oxide film on the surface after removal of the chemical oxidefilm, whereby the thermal oxide film is the only film on the surface. 8.The method of claim 4, further comprising: forming a thermal oxide filmon the surface after removal of the chemical oxide film, whereby thethermal oxide film is the only film on the surface.